Combline filter

Abstract

 A combline bandpass filter (100) has a notch filter structure (108c, C3) at its output to introduce a notch into its passband. The notch structure is an inductive transmission line element (108c) in parallel with a capacitor (C3), preferably provided by a RAMPT device.

Description

[0001] The present invention relates to a combline bandpass filter.

[0002] In the field of portable communication equipment such as portable telephones, size and manufacturing cost are of great concern. Such requirements may not only be concerned with portable communication equipment. In this regard, development of various techniques meeting those requirements has been actively made.

[0003] One method for achieving a reduction in size is to use a configuration, which can be implemented in a limited space, such as transmission lines (striplines or micro striplines), in place of a passive element occupying a large area or volume. A representative example of such a configuration is a filter implemented using transmission lines (striplines or micro striplines) to have a filter function for extracting signals of a desired frequency band while attenuating noise signals of other frequency bands. Such a filter may be used in various fields including radio communication systems. In radio communication systems, the filter can be used for a receiver for selecting desired signals or for a transmitter for reducing radiated out of band signals, such as harmonics and unwanted mixing products.

[0004] An example of a conventional stripline filter is disclosed in US-A-4963843. The combline stripline filter, disclosed in this document, uses conductive strips, each connected to the ground at one end, and capacitively loaded at the other end. That is, the combline stripline filter includes a substrate having top and bottom surfaces each forming a ground plane. An inner circuitry layer is formed between the top and bottom surfaces of the substrate. The combline stripline filter also includes a ground area having a plurality of angled edges coupled to the ground planes. The inner circuitry layer is formed by combline resonators each coupled to the ground at one end and capacitively coupled to the ground at the other end. This combline stripline filter uses pattern capacitors in that the combline resonators are arranged in an interlayered fashion.

[0005] However, such a stripline filter, which uses pattern capacitors having the above mentioned structure, suffers from the problems of undesirably large layout size and an increased error rate occurring in the pattern capacitors due to interference. Furthermore, it is difficult to connect the stripline filter to other devices. Where the pattern capacitors are capacitively loaded to the ground, it is difficult to accurately calculate the loaded capacitance. Since the capacitance between each pattern capacitor element and the ground may vary depending on the material of the substrate, difficulties can arise in the manufacture of such a stripline filter. Furthermore, this stripline filter has a restriction in terms of its size and position when it is connected to other devices. This is because the connection of each pattern capacitor to an input/output pad and the ground is made at ends of the substrate.

[0006] Similar to a general filter using a passive element, the above mentioned filter using transmission lines has a desired frequency bandwidth for its filtering operation. The frequency bandwidth is determined by the space between adjacent transmission lines, the width of each transmission line, and the capacitance of the pattern capacitors coupled to the transmission lines.

[0007] In pace with recent developments in the communication industries, more subdivided frequency bands have been used. However, this causes a reduction in the width between adjacent allocated frequency bands. As a result, the allocated frequency bands may interfere with each other. For this reason, it may be impossible to provide radio services of a good quality. Where the filter uses a reduced frequency bandwidth in order to reduce the interference between the allocated frequency bands, another problem of a reduction in the gain of the filter occurs even though the interference is reduced.

[0008] A combline filter according to the present invention is characterised by an output notch filter structure.

[0009] Preferably, the output notch filter structure comprises an inductive transmission line element in parallel with a capacitive element. More preferably, said inductive transmission line element and said capacitive element are on opposite sides of a substrate and/or said capacitive element is provided by a RAMPT device.

[0010] Preferably, a filter according to the present invention comprises first and second F-shaped transmission line structures on a substrate, the second F-shaped structure being rotated by 180° with respect to the first F-shaped structure and the F-shaped structures only lying adjacent and parallel in their regions between their arms. More preferably, the notch filter structure comprises an inductive transmission line element extending from the lower arm of the second F-shaped structure. Yet more preferably, said inductive transmission line structure is L-shaped, the toe of the L being coupled to distal end of the lower arm of the second F-shaped structure and the "upright" of the L lying parallel to the lower part of the "upright" of the second F-shaped structure.

[0011] In the case of the filter comprising F-shaped transmission line structures, it is preferable that the input of the filter is at the distal end of the lower arm of the first F-shaped structure, the distal end of the upper arm of the first F-shaped structure is connected to ground via a capacitor and the foot of the first F-shaped structure is connected directly to ground and/or the distal end of the upper arm of the second F-shaped structure is coupled to ground via a capacitor and the foot of the second F-shaped structure is coupled directly to ground.

[0012] An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a view illustrating the pattern of a transmission line filter according to the present invention;

Figure 2 is a circuit diagram illustrating the lumped circuit corresponding to the pattern of the radio frequency filter shown in Figure 1; and

Figure 3 is a graph showing the characteristics of the transmission line filter of Figure 1.

[0013] In accordance with the present invention, a radio frequency filter is implemented using transmission lines. Where a radio frequency filter is implemented using transmission lines, for which striplines or micro striplines for example may be used, the design thereof may be varied depending on the kind of the transmission lines used.

[0014] Generally, such a radio frequency filter using transmission lines has a multilayered structure. The multilayered structure of the radio frequency filter may be varied depending on whether the radio frequency filter uses striplines or micro striplines for its transmission lines. For example, where the radio frequency filter uses micro striplines, it has a multilayered structure having two layers. However, where striplines are used for the transmission lines, the radio frequency filter has a multilayered structure having three layers.

[0015] First, the multilayered structure of a radio frequency filter designed using micro striplines will be described. In this case, a ground layer is arranged as a lower layer of the multilayered structure, whereas a filter layer having a designed pattern is arranged as an upper layer of the multilayered structure. The pattern is connected to the lower layer, that is, the ground layer, through via holes, or coupled to a capacitance compensating circuit through via holes.

[0016] On the other hand, the multilayered structure of a radio frequency filter designed using striplines further has an another layer arranged on the filter layer of the multilayered structure in the above mentioned radio frequency filter designed using micro striplines. That is, in this multilayered structure, ground layers are disposed, as upper and lower layers, over and beneath the filter layer having a pattern designed using micro striplines. The upper ground layer is provided with a pattern corresponding to output and input terminals, and a pattern corresponding to a capacitance compensating circuit.

[0017] Although the radio frequency filter has a multilayered structure varying depending on whether it uses striplines or micro striplines for its transmission lines, as mentioned above, the pattern of its filter layer is the same in either case. Accordingly, the following description associated with a preferred embodiment of the present invention will be given irrespective of which type of transmission lines are used. That is, only the pattern of transmission lines will be illustrated.

[0018] Referring to Figure 1, a radio frequency filter has a combline structure and includes a frequency cut-off circuit according to the present invention. That is, Figure 1 shows the pattern structure of a radio frequency filter using transmission lines in accordance with the present invention.

[0019] Referring to Figure 1, a filter layer 100 has a combline structure forming a radio frequency filter on a copper clad laminate (CCL) substrate. The structure comprises first, second and third transmission lines 108a, 108b, 108c. The transmission lines 108a, 108b, 108c are connected to ground through first to seventh via holes 110, 112, 114, 116, 118, 120, 122. More particularly, the first and second transmission lines 108a, 108b are connected to a lower ground layer through the first and sixth via holes 110, 120. The first and second transmission lines 108a, 108b are also coupled, through the third and fourth via holes 114, 116, to capacitance compensating circuits connected to ground.

[0020] The first transmission line 108a is also coupled, through the second via hole 112, to an input terminal connected to ground. The transmission line 108b is coupled, through the fifth via hole 118, to an output terminal connected to ground. In order to implement a frequency cut-off circuit in accordance with the present invention, the radio frequency filter has the necessary inductance and capacitance. The inductance is determined by the length of the third transmission line 108c, i.e. ℓ7 + ℓ8. For the capacitance, the radio frequency filter is provided with a separate capacitive element. To this end, the third transmission line 108c is connected, through the seventh via hole 122, to a capacitive element coupled to ground. Such a structure is called a "blind via hole" structure. Alternatively, the third, fifth and seventh via holes 114, 116, 122 may extend to the lower ground layer so as to connect the capacitance compensating circuits to the lower ground layer. This structure is called a "through via hole". In the present example, the blind via hole structure is used.

[0021] The first and second transmission lines 108a, 108b of the radio frequency filter form one transmission line pair. The first transmission line 108a is connected to an input terminal and the second transmission line 108b is connected to an output terminal. The first to sixth via holes 110, 112, 114, 116, 118, 120 are formed at ends of the first and second transmission lines 108a, 108b and the input and output terminals. The first and sixth via holes 110, 120 connect the respective associated transmission lines 108a, 108b to the ground layer, whereas the third and fourth via holes 114, 116 connect the associated transmission lines 108a, 108b to capacitance compensating circuits. Each of the capacitance compensating circuits is implemented using the capacitor of a RAMPT circuit. The capacitance of each capacitance compensating circuit an appropriate value for the frequency band to be filtered. The capacitance compensating circuits are used to allow the transmission lines reduced lengths while allowing easy impedance matching and tuning. In particular, such easy impedance matching and tuning is possible by use of the capacitor of a RAMPT device having an appropriate capacitance without any adjustment of width or distance to achieve an adjustment in capacitance as in conventional cases. Although the capacitance compensating circuits are illustrated in Figure 1 as being formed at the corresponding ends of the first and second transmission lines 108a, 108b, this positioning will depend on the structure of the radio frequency filter to be implemented. The stray capacitances of the third and fourth via holes 114, 116 should also be taken into consideration in determining the capacitances of the capacitance compensating circuits. Since each of the third and fourth via holes 114, 116 has a certain capacitance, this capacitance has to be reflected in the capacitance of the associated capacitance compensating circuit. The via hole structure of the radio frequency filter should also be taken into consideration in reflecting respective capacitances of the third and fourth via holes 114, 116. This is because the capacitances of the third and fourth via holes 114, 116 differ depending on the type of via hole used, i.e. blind or through via hole structures.

[0022] The seventh via hole 122 connects the third transmission line 108c to a capacitive element included in a frequency cut-off circuit. Hereinafter, this transmission line 108c is referred to as an "inductive transmission line". In order to achieve cutting-off of a specific frequency using the frequency cut-off circuit, it is necessary to determine an appropriate length for the inductive transmission line 108c. In Figure 1, the length of the inductive transmission line 108c is indicated by ℓ7 + ℓ8. That is, the inductive transmission line 108c extends from a point, where the inductive transmission line is connected to the output terminal through the fifth via hole 118, by the length of ℓ7 + ℓ8. This inductive transmission line may have a bent structure as shown in Figure 1, in order to reduce the size of the radio frequency filter. Once the cut-off frequency has been determined, it is possible to establish the length of the inductive transmission line, based on a value obtained by a calculation based on the determined frequency along with a value experimentally obtained.

[0023] For example, the cut-off frequency can be determined using the following Equation 1:

where, "f" represents the cut-off frequency, "L" represents an inductance, and "C" a capacitance.

[0024] As described above, the capacitive element connected to the inductive transmission line 108c through the seventh via hole 122 may be configured using the same type of element as that used in the capacitance compensating circuit. That is, the capacitive element may be implemented using the capacitor of a RAMPT device, as in the capacitance compensating circuit. The capacitance of the capacitive element is coupled to the inductance of the inductive transmission line 108c such that the cut-off frequency is the desired cut-off frequency. As described above, the inductance of the inductive transmission line 108c is determined by its length. Accordingly, the inductance of the inductive transmission line 108c can be appropriately determined in order to set a desired cut-off frequency. In other words, under the condition in which a desired inductance L and a desired cut-off frequency are determined, the capacitance C of the capacitive element can be determined by applying the determined values to Equation 1.

[0025] Where the RAMPT device is used for the capacitive element, it is possible to appropriately adjust the capacitance C of the capacitive element, if necessary. In this case, therefore, it is possible to vary the cut-off frequency. This is apparent by referring to Equation 1.

[0026] The capacitance of the capacitive element should be determined, taking into consideration the capacitance possessed by the seventh via hole 122, as in the case in which the capacitance of each capacitance compensating circuit is to be determined. In this case, in determining the capacitance of the capacitive element, the capacitance possessed by the seventh via hole 122 should be taken into consideration, as in the case of determining the capacitance of the capacitance compensating circuits. In this case, whether the radio frequency filter uses blind via hole structures or through via hole structures should also be taken into consideration in determining the capacitance of the seventh via hole 122.

[0027] Referring to Figure 2, it can be seen that the first to sixth via holes 110, 112, 114, 116, 118, 120, 122, first, second and third capacitive elements c1, c2, c3, and input and output terminals 210, 212 are connected to the first and second transmission lines 108a, 108b associated therewith.

[0028] The first via hole 110 connects the first transmission line 108a to ground and the sixth via hole 120 connects the second transmission line 108b to ground. The fourth via hole 116 connects the first transmission line 108a to ground via the first capacitance compensating circuit c1 and the third via hole 114 connects the second transmission line 108b to ground via the second capacitance compensating circuit c2. The first transmission line 108a is connected to the input terminal through the second via hole 112, whereas the second transmission line 108b is connected to the output terminal through the fifth via hole 118.

[0029] The seventh via hole 122 connects the second transmission line 108b to ground via the third capacitive element c3. In Figure 2, "a" to "f" represent points where the transmission lines are bent or branched.

[0030] Referring to Figure 3, it can be seen that the frequency band of the radio frequency filter exhibits a reduction in gain at the cut-off frequency set by the frequency cut-off circuit. It can also be found that the cut-off frequency is determined by inductance L and capacitance C. That is, the radio frequency filter allows frequencies of a low band in a given frequency band to pass by virtue of the inductance L while allowing frequencies of a high band in the give frequency band to pass by virtue of the capacitance C. Accordingly, where the characteristic graphs based on the inductance L and capacitance C, and the characteristic graph of the radio frequency filter are simultaneously applied, a reduction in gain occurs at a specific frequency in the given frequency band by virtue of the characteristic graphs of the inductance L and capacitance C. Thus, it is possible to prevent interference among similar frequency bands by the frequency cut-off circuit according to the embodiment of the present invention, which cuts off a specific frequency.

[0031] The operation of the radio frequency filter having the above mentioned configuration according to the embodiment of the present invention will be described in detail.

[0032] The radio frequency filter filters signal components of a specific frequency band from a signal applied thereto at the input terminal 210 and outputs the resultant signal at the output terminal 212. The specific frequency band is determined by the capacitances of the first and second capacitance compensating circuits c1, c2 and the space between the first and second transmission lines 108a, 108b. The signal output after the filtering of the signal, applied to the radio frequency filter at the input terminal 210, in the specific frequency band is shown in Figure 3.

[0033] Referring to Figure 3, it can be seen that a considerable gain reduction occurs at a specific frequency in the specific frequency band. It can also be seen that the cut-off frequency is set to about 2.20 GHz. As described hereinbefore, the cut-off frequency is determined by the inductance given by the length of the third transmission line 108c corresponding to ℓ7 + ℓ8 and the capacitance given by the third capacitive element c3. That is, among signals of the specific frequency band filtered by the radio frequency filter, those of the cut-off frequency are cut off by virtue of the third transmission line 108c having a length of ℓ7 + ℓ8 and the third capacitive element c3. Accordingly, only the signals of the specific frequency band, from which the signals of the specific cut-off frequency are cut off, are output.

[0034] Although the radio frequency filter has a configuration for cutting off a frequency at one side of the specific frequency band in the above described embodiment of the present invention, it is possible to implement a configuration capable of cutting off specific frequencies at opposite sides of the specific frequency band. It will also be appreciated that a configuration capable of cutting off a frequency at a higher frequency side of the specific frequency band.

[0035] As apparent from the above description, the present invention provides a radio frequency filter capable of cutting of frequencies having a possibility of adversely affecting the frequency band to be used, thereby achieving an improvement in the quality of radio communication services. In accordance with the present invention, capacitive elements are comprised of RAMPT elements. Accordingly, it is possible to adjust the frequency to be cut off.

Claims

1. A combline bandpass filter characterised by an output notch filter structure (108c, C3).

2. A filter according to claim 1, wherein the output notch filter structure comprises an inductive transmission line element (108c) in parallel with a capacitive element (C3).

3. A filter according to claim 2, wherein said inductive transmission line element (108c) and said capacitive element (C3) are on opposite sides of a substrate.

4. A filter according to claim 2 or 3, wherein said capacitive element (C3) is provided by a RAMPT device.

5. A filter according to any preceding claim, comprising first and second F-shaped transmission line structures (108a, 108b) on a substrate, the second F-shaped structure (108b) being rotated by 180° with respect to the first F-shaped structure (108a) and the F-shaped structures (108a, 108b) only lying adjacent and parallel in their regions between their arms.

6. A filter according to claim 5, wherein the notch filter structure (108c, C3) comprises an inductive transmission line element (108c) extending from the lower arm of the second F-shaped structure (108b).

7. A filter according to claim 6, wherein said inductive transmission line structure (108c) is L-shaped, the toe of the L being coupled to distal end of the lower arm of the second F-shaped structure (108b) and the "upright" of the L lying parallel to the lower part of the "upright" of the second F-shaped structure (108b).

8. A filter according to claim 6 or 7, the input (210) of the filter is at the distal end of the lower arm of the first F-shaped structure (108a), the distal end of the upper arm of the first F-shaped structure (108a) is connected to ground via a capacitor (C1) and the foot of the first F-shaped structure (108a) is connected directly to ground.

9. A filter according to claim 6, 7 or 8, wherein the distal end of the upper arm of the second F-shaped structure (108b) is coupled to ground via a capacitor (C2) and the foot of the second F-shaped structure (108b) is coupled directly to ground.

10. In a radio frequency filter of a combline structure including an input terminal, an output terminal, transmission lines arranged in pair, each of the transmission lines having a desired width while being connected to a capacitance compensating circuit through a via hole, whereby the radio frequency filter has a predetermined frequency bandwidth, a frequency cut-off circuit for cutting off a specific frequency from a frequency band having the predetermined frequency bandwidth, the frequency cut-off circuit comprising:

an inductive transmission line extending from the output terminal by a length determined to provide an approximate inductance corresponding to a calculated value approximate to an inductance for obtaining the specific frequency; and

a capacitive element coupled to the approximate inductance provided by the inductive transmission line, so that it has a capacitance for obtaining the specific frequency;

   wherein the inductive transmission line is connected to the capacitive element through a via hole formed at an end of the transmission line opposite to the output terminal, from which the transmission line extends.

11. The frequency cut-off circuit according to claim 10, wherein the capacitive element is a capacitor of a RAMPT device.

12. The frequency cut-off circuit according to claim 10, wherein each of the transmission lines is a micro stripline.

13. The frequency cut-off circuit according to claim 10, wherein each of the transmission lines is a stripline.

14. The frequency cut-off circuit according to claim 10, wherein the inductive transmission line is bent at a desired bending length ratio.

15. The frequency cut-off circuit according to claim 10, wherein the inductance of the inductive transmission line and the capacitance of the capacitive element are calculated, based on the following Equation:
[Equation]

where, "f" represents the cut-off frequency, "L" represents the inductance, and "C" the capacitance.

16. In a radio frequency filter of a combline structure including an input terminal, an output terminal, transmission lines arranged in pair, each of the transmission lines having a desired width while being connected to a capacitance compensating circuit through a via hole, respectively, whereby the radio frequency filter has a predetermined frequency bandwidth, a method for cutting off a specific frequency from a frequency band having the predetermined frequency bandwidth, comprising the steps of:

calculating an inductance approximate to an inductance for obtaining the specific frequency;

determining a length of the inductive transmission line to extend from the output terminal, based on the approximate inductance; and

calculating a capacitance of a capacitive element coupled to the approximate inductance provided by the inductive transmission line while being connected to the inductive transmission line through a via hole to obtain the specific frequency.

17. The method according to claim 7, wherein the inductance of the inductive transmission line and the capacitance of the capacitive element are calculated, based on the following Equation:
[Equation]

where, "f" represents the cut-off frequency, "L" represents the inductance, and "C" the capacitance.

Drawing